1. Field of the Invention
The present invention relates to a frequency synthesizer capable of supporting multifrequency bands. The present invention also relates to a digital radio communication system capable of supporting a plurality of frequency bands, which transmit image and speech information using digital data.
2. Description of the Related Art
In recent years, with a multifrequency bands requirement of a digital radio communication system, a technique using a frequency synthesizer capable of supporting multifrequency bands has been proposed.
In addition, when a plurality of frequency synthesizers or the frequency synthesizer capable of supporting the multifrequency bands is used, a digital radio communication system capable of supporting multifrequency bands can be configured.
Hereafter, referring to FIG. 5 and Japanese Laid-Open Patent Application Publication No. 2002-290235, a configuration of the frequency synthesizer capable of supporting the multifrequency bands will be explained as a prior art.
FIG. 5 is a block diagram showing a high frequency unit of the prior art of the digital radio communication system which uses the frequency synthesizer capable of supporting the multifrequency bands.
In FIG. 5, intermediate-frequency voice data inputted from a voice data signal terminal 25 is modulated by a modulation unit 24 and is inputted into a frequency converting unit 23. In the frequency converting unit 23, using a local oscillation signal of a voltage controlled oscillator 28 of a first frequency synthesizer unit 29 or a local oscillation signal of a voltage controlled oscillator 30 of a second frequency synthesizer unit 31, the intermediate-frequency voice data are frequency-converted.
Here, the local oscillation signal of the voltage controlled oscillator 28 and the local oscillation signal of the voltage controlled oscillator 30 have different frequencies. A selection whether the voltage controlled oscillator 28 is used or the voltage controlled oscillator 30 is used is performed by a switch 27. Meanwhile, a local oscillation signal selected by the switch 27 is inputted into the frequency converting unit 23 through a switch 26 for switching transmission and reception.
An output signal of the frequency converting unit 23 is then amplified to a specified output power level by a driver unit (DRV) 22 and a power amplifier unit (PA) 21. An output signal of the power amplifier unit 21 is transmitted from an antenna 19 through a switch 20 for switching transmission and reception. At this time, multifrequency bands can be supported based on switching to the local oscillation signal of the voltage controlled oscillator 28 or the local oscillation signal of the voltage controlled oscillator 30, each of which has different frequency.
The switch 20 switches between a state of supplying the output of the power amplifier unit 21 to the antenna 19 as a transmission signal and a state of supplying a signal received by the antenna 19 to a front-end unit 32.
When the received signal is inputted through the switch 20 from the antenna 19, the received signal enters the front-end unit 32. In the front-end unit 32, a desired received signal is amplified, selected by a filter, which is not shown, and is inputted into a frequency converting unit 33.
The received signal inputted into the frequency converting unit 33 is frequency-converted using the local oscillation signal of the voltage controlled oscillator 28 or the local oscillation signal of the voltage controlled oscillator 30. A selection whether the voltage controlled oscillator 28 is used or the voltage controlled oscillator 30 is used is performed by a switch 27. A local oscillation signal selected by the switch 27 is then inputted into the frequency converting unit 33 through the switch 26 for switching transmission and reception.
An intermediate-frequency received signal converted by the frequency converting unit 33 is demodulated by a demodulation unit 34 and is outputted from a voice data signal terminal 35 as an audio signal.
Here, in the digital radio communication system which performs a transmission of the transmission signal and a transmission of the received signal alternately, the local oscillation signal is switched for the transmission signal or for the received signal by the switch 26, and inputted into the frequency converting unit 23 or the frequency converting unit 33.
Meanwhile, according to a frequency synthesizer disclosed by Japanese Laid-Open Patent Application Publication No. 2002-290235, it includes a plurality of prescalers, a plurality of multipliers, and a switching unit, wherein an output signal of the multiplier is used as a local oscillation signal, and a combination of a desired prescaler and multiplier is set so that a product of a division ratio and a multiplication number may become constant. According to this configuration, a variable frequency step of a synthesizer output frequency can be kept at a constant value regardless of a frequency band to be selected. In addition, according to this frequency synthesizer, a prescaler which is not selected by switching a plurality of prescalers is turned off, thereby achieving a reduction in power dissipation.
However, in the digital radio communication system using the frequency synthesizer capable of supporting the multifrequency bands shown in FIG. 5, a frequency synthesizer unit is individually needed for each of a plurality of frequency bands, resulting in a problem of an increase in mounting area of the frequency synthesizer unit.
Meanwhile, another prior art is disclosed in the Japanese Laid-Open Patent Application Publication No. 2002-290235. However, according to this prior art, a plurality of multipliers are needed, resulting in a problem of an increase in mounting area and an increase in power consumption of the multiplier.
In a configuration in which the output of the voltage controlled oscillator is used as the local oscillation signal not by using the multiplier but by varying the division ratio of the divider, a loop gain characteristics of the frequency synthesizer unit is different for every different oscillation frequency. As a result, there may arise a problem that a difference will occur in a lock-up time characteristic and a C/N characteristic, which are frequency synthesizer characteristics.